As a display means of a large-sized image, various projection type image display devices using liquid crystal elements have been developed recently. In addition, image display devices employing cathode ray tubes have been used. An example of such a display device is described in "LCD Full-Color Video Projection", Society for Information Display, Digest 1986, PP. 375-378 which is incorporated herein by reference. The structure of the display device is shown in FIG. 9. As is shown, the light emitted by the halogen lamp 51, after being polarized by reflector 52 and condenser lens 53, is separated into light of red color, green color, and blue color by dichroic mirrors 54 and 55, and is projected onto the respective liquid crystal display elements 60, 61, and 62 for red, green and blue. The green light is projected directly onto its corresponding display element while the red and blue color lights are projected onto their corresponding display elements after being reflected by mirrors 56, 57, 58, and 59. The liquid crystal display elements 60, 61, and 62 each consist of plural picture elements whose light transmissivities are controlled electrically and optically according to an image signal (not shown) from the outside to act as light shutters and to form an image. The images are synthesized by a dichroic prism 63 enlarged and projected onto a screen 65 by a projection lens 64 to produce a colored large image.
A similar projection type image display device is disclosed in the specification of Open-laying gazette No. Hei3-291644 of Japanese patent application. In the device, white light from a source is divided into color light by an inclined type dichroic mirror for blue and green, controlled in strength by a light valve, synthesized and projected.
As the light source for such a projection type display device with liquid crystal elements, short arc type xenon lamps or metal halide lamps as well as halogen lamps are generally used. In recent times, metal halide lamps have been employed frequently due to their superior characteristics with regard to geometrical compactness, favorable spectrum generation and high efficiency.
In a projection type image display device with only one light source, the white light emitted by the light source is divided into red, green, and blue lights by a light analyzing system of a dichroic mirror or dichroic prism, projected onto liquid crystal display elements, and synthesized by a color-synthesizing optical system. The necessity for both the color analyzing system and color synthesizing system has been an obstacle for reduction of size and cost of the conventional devices.
Also, the image display device having a metal halide lamp as its light source exhibits disadvantageous color shift. A metal halide lamp including metal elements emitting a few strong line spectrum (such as Tl, Li, or In) and metal elements emitting a few weak line spectrum as well as a few strong line spectrum (such as Dy, Nd, Ho, or Tm) are known to have a ratio of light emission which varies as time passes during long periods of operation. For example a metal halide lamp including TlI, LiI, and InI suffers, after about 800 hours of operation, variation of color due to change of strength ratio of red, green, and blue lights. One report has noted a particular decrease of red light.
Further, to coincide the chromaticity of a projected color image with a specified standard, the amounts of metal elements in the lamp and the optical properties of the optical parts (the dichroic mirror, for example) combined with the lamp had to be designed with extreme care, since the chromaticity of the lamp with several metals is not adjustable at will.
Furthermore, the power efficiency of the conventional system was low, because the excessively strong part of the light spectrum is desireably suppressed to obtain a desired chromaticity in concert with the weak part of the light spectrum.
On the other hand, projection type liquid crystal display devices with so called single-plate-type liquid crystal have been developed, which are provided with color filters for red, green, and blue for the picture elements. In such a liquid crystal display device, however, the red light-transmitting picture element, for example, utilizes red light only from the white light of the source. The situation is similar for the green or blue light-transmitting elements. Thus, the efficiency of light utility was very low and insufficient for high luminance or high quality.
There has been disclosed a display device to overcome the above described defects in the Japanese Patent Open-laying Gazette No. Hei3-56922 is shown in FIG. 10. The light source of the device consists of three cathode ray tubes 66, 67, and 68 each emitting red, green, and blue light respectively. The red, green, and blue lights are made parallel by a condenser lens 69 and projected onto a liquid crystal display element 71 through a microlens array 70. The liquid crystal display element 71 functions as an optical shutter to change the transmittance of each picture element in accordance with the outer signal, and provides a color image, which is enlarged on a screen 73 by a projection lens 72. This device having CRTs 66, 67, and 68 of single color as the light source can have red, green, and blue lights without energy loss due to unbalance between the light sources. To obtain a projected image with more precision and of high luminance, however, the number of picture elements and the luminance of CRTs is desireably increased.